1. Field of Invention
This invention pertains to method and apparatus for reading information stored in helical stripes on magnetic medium, and particularly to method and apparatus for tracking servo signals embedded in helical stripes on magnetic tape in order to recover errors that occur when reading helically recorded magnetic tape having distorted tracks.
2. Prior Art and Other Considerations
Numerous prior art patents teach the recording and reading of information stored in helical stripes on magnetic tape. In a helical scan arrangement, travelling magnetic tape is partially wrapped around a rotating drum so that heads positioned on the drum are contiguous to the drum as the drum is rotated. A write head on the drum physically records data on the tape in a series of discrete stripes oriented at an angle with respect to the direction of tape travel. The data is formatted, prior to recording on the tape, to provide sufficient referencing information to enable later recovery during readout.
Among such prior art teachings are the following United States Patents, all commonly assigned herewith and incorporated herein by reference:
(1) U.S. Pat. No. 4,843,495 to Georgis et al., entitled CYCLICAL SERVO ZONE TRACKING METHOD AND APPARATUS FOR HELICAL SCAN RECORDING DEVICES;
(2) U.S. Pat. No. 4,835,638 to Hinz et al., entitled APPARATUS AND METHOD FOR FORMATTING AND RECORDING DIGITAL DATA ON MAGNETIC TAPE; and,
(3) U.S. Pat. No. 4,845,577 to Georgis et al., entitled APPARATUS AND METHOD FOR ENABLING RAPID SEARCHING OF HELICALLY RECORDED MAGNETIC TAPE.
Helical scan recorders require that the read and write heads be correctly aligned with the stripes of recorded information on the magnetic tape. To this end, servo signals, sometimes referred to as "pilot" signals, are embedded in servo "zones" in each stripe. Typically there are a plurality of servo zones on each stripe.
In some prior art helical scan systems, such as U.S. Pat. No. 4,843,495 to Georgis et al. referenced above, a read head dually functions to detect servo signals provided in a stripe and to read the informational data provided in the stripe. Upon detection of the servo signals by the read head, the system determines if the position of the heads on the drum need be adjusted for correct alignment with the stripes.
In other prior art helical scan systems, a special servo head is provided on the drum to detect the servo signals and determine whether head adjustment is needed. A plurality of servo zones are sometimes provided in each stripe. In these prior art systems, servo heads typically attempt to evenly strattle two adjacent stripes. The amplitudes of the signals derived from the servo zones of the two adjacent stripes are compared. A positional adjustment of the servo head is made to equalize the amplitudes of the servo signals from the two stripes. Examples of such dual azimuth helical scan servo arrangements having such features include U.S. Pat. No. 4,528,605 to Hiraguri, U.S. Pat. No. 4,581,662 to Sato, and U.S. Pat. No. 4,665,447 to Odaka.
It is well known that operational problems may cause one or more tracks of information, helically recorded on magnetic tape storage media, to appear upon playback or reading as a distorted track.
One type of distorted track is a curved track. In this respect, problems such as those associated with the handling or guiding of a magnetic tape as it is being read may cause a track to appear as a curved track.
Various prior art schemes have been developed to compensate for the reading of curved tracks. These schemes primarily enable a track-reading head or transducer to follow the curvature of the track. Typically this is done by mounting the track-reading head upon an element (such as a bi-morph leaf) that can be deflected to permit the head to follow the curved track. Examples of such schemes are provided in U.S. Pat. No. 4,486,796 to Sakamoto; U.S. Pat. No. 4,420,778 to Sakamoto; U.S. Pat. No. 4,404,605 to Sakamoto; U.S. Pat. No. 4,172,265 to Sakamoto et al.; U.S. Pat. No. 4,099,211 to Hathaway; and, U.S. Pat. No. 4,106,065 to Ravizza.
Accordingly, it is an object of the present invention to provide servo tracking method and apparatus for allowing of data recovery when reading distorted tracks, such as curved tracks.